The present invention relates to data transmission. It concerns the recovery at a receiver of a clock rate from a synchronous digital signal transmitted using linear modulation.
In a data transmission system using linear modulation, a synchronous binary signal is shaped by a coder before being modulated and transmitted. This shaping is to limit the pass band of the transmitted signal, and it transforms the synchronous binary signal into a signal having a plurality of levels giving a spectrum without any component at the clock rate, so that it is not possible to recover the clock rate at a receiver simply by filtering the received and demodulated signal, and it is necessary to use non-linear processing of the demodulated signal.
Clock rate recovery circuits are known comprising an oscillator which is centred on the clock rate frequency and is servo-controlled by a phase locked loop operating on the output signal from a full-wave rectifier circuit which receives the demodulated signal. Such circuits have the drawback of tolerating only a small phase difference between the carriers of the transmitter and of the receiver.